The present embodiments relate to a method for determining sensitivity matrices for hotspots for a magnetic resonance tomography device.
Magnetic resonance tomography devices may include at least one basic magnet, a radio-frequency transmit system and a radio-frequency receive system. Using the basic magnets, a temporally constant and spatially essentially homogeneous magnetic field is built up in a volume under examination. Radio-frequency transmission pulses are applied by the radio-frequency transmit system to the volume under examination. The pulses excite an object under examination, which has been introduced into the volume under examination, to magnetic resonances. The excited magnetic resonances are received by the radio-frequency receive system. The radio-frequency transmit system and the radio-frequency receive system may be devices separated from one another. As an alternative, a combined transmit and receive system may be used.
The magnetic resonance tomography device may also include gradient systems to enable the magnetic resonance tomography device to perform local encoding of the magnetic resonance signals.
The radio-frequency transmit system may have a plurality of antenna elements, to which a corresponding control device is operable to apply corresponding control signal sequences individually. During operation of the radio-frequency transmit system, in which the control signal sequence for each antenna element may have any given pulse form (e.g., variation of amplitude and phase), numerous possibilities for overlaying of the electrical fields in the body arise. This produces a very high level of complexity if the local Specific Absorption Rate (SAR) is to be monitored or calculated in advance for given pulses. Local SAR is monitored for the safety of the object under examination (e.g., the patient) and with local coils, is also demanded by corresponding regulations. The electrical fields overlaying each other in a radio-frequency system with a number of individually-controllable antenna elements are monitored because the electrical fields add to each other in a linear manner, but the local power release is proportional to the square of the resulting electrical field.
The local SAR is not directly measurable. Therefore, body models with (complex) conductivity distribution may be created, and with the models, the fields caused by the respective antenna elements may be calculated at individual points of the model. Such calculations may be performed with the Finite Differential Time Domain (FDTD) method.
The object under examination may be divided up into a plurality of voxels. For each voxel of the plurality of voxels, the electrical field strengths caused by the individual antenna elements and the overlaying of the antenna elements are established. The number of voxels observed is large. With some models, 50,000 and in some cases, far above 100,000 voxels (e.g., several million voxels in extreme cases), are observed.
In view of the complexity of the calculations to be performed and the plurality of calculations to be made, real-time monitoring or an online capability is out of the question with such a procedure. The aim is thus to reduce the computing outlay.
A method is described in DE 10 2009 024 077.2 for determining a main receptor point voxel, in which the electrical field reaches an absolute maximum based on the electrical fields generated. The phase relationships of the individual antenna elements, at which the absolute maximum is reached, are determined. Phase relationships that differ from the phase relationships of the individual antenna elements (e.g., new phase relationships), at which the absolute maximum is reached, are selected. For the new phase relationships, at least one additional receptor point voxel, in which the resulting electrical field reaches a relative maximum, is determined. In the subsequent procedure, both the main receptor point voxel and the at least one additional receptor point voxel are stored, and the electrical field is monitored both in the main receptor point voxel and also in the at least one additional receptor point voxel.
This method already represents a significant advance in relation to the conventional prior art. However, with this method, a critical SAR may be exceeded for a given control signal sequence at a point of the object.
In DE 10 2009 030 721.4, a method for SAR determination, in which monitoring is undertaken at selected hotspots, with the number of hotspots moving in the range of between 100 and 1000 is described. The hotspots are determined on the basis of empirical values.
This procedure also represents a significant advance. This procedure has online capabilities and under some circumstances, real-time capabilities. The procedure described in DE 10 2009 030 721.4, however, depends on the hotspots being correctly selected so that SAR values that are too high may be safely avoided at points other than the monitored points of the object. DE 10 2009 030 721.4 does not provide any information on the correct selection of the hotspots.